This invention relates to signal detection and, more particularly, to channel tracking techniques in mobile receivers adapted to receive phase-modulated signals.
One well-known technique for transmitting information to mobile receivers is to convert the signal to digital symbols, to map those symbols onto a two-dimensional space, to modulate a carrier with the mapped symbols, and to transmit the modulated carrier to the receiver. The modulation of a symbol mapped onto a two-dimensional space (having x and y coordinates) takes place by amplitude-modulating the x component of the symbol by a carrier signal, amplitude-modulating the y component of the symbol by the carrier signal shifted by 90 degrees, and adding the two modulation products. In some applications the mapping is restricted to a circle, and that, effectively, results in phase modulation of the carrier.
A mobile unit receives signals that are corrupted by inter-symbol interference (ISI) as well as by thermal noise, and the challenge is to detect the so-distorted symbols. The ISI is a non-stationary process when the mobile unit is moving. That is, the characteristics of the channel are based on the location of the mobile unit relative to the transmitter, and when that location changes, the channel characteristics change. Prior art systems allow for adapting a receiver's response to the channel characteristics, but this adapting requires processing, and the processing requires time. As long as the channel characteristics change slowly, there is no problem. When the channel characteristics change rapidly, such as when the mobile unit changes its location rapidly (e.g. the mobile unit is in a car, or a plane), the currently-used adapting processes are able to keep up with the changes under ideal conditions.
The challenge to track the changing channel characteristics is compounded by the fact that the mobile unit has no information about the precise time when symbols are applied to the transmitter's modulator, and therefore does not know precisely when to sample the received signal. Furthermore, although the receiver nominally knows what the transmitter's carrier frequency is, the actual carrier frequency may be off and, in any event, the receiver's local frequency may be off from its specified value because of normal manufacturing tolerance issues, temperature variations, etc.
When the receiver's local oscillator is not equal to the transmitter's oscillator, an offset in frequency is said to exist. When there is no offset in frequency, the received signal is sampled, converted to digital form, and applied to a detection algorithm. The detection algorithm must remove the ISI introduced by the channel and must also compensate for the changing characteristics of the channel due to the movements of the mobile unit (e.g., in a car moving at 60 miles per hour, the channel characteristics change fairly rapidly). One technique that accomplishes channel tracking is the Least-Mean-Squared (LMS) algorithm. The LMS algorithm, however, is not thought of as being able to handle changing channel characteristics when there is a significant frequency offset.
When the receiver's frequency does have a significant offset, conventional differential detectors can be used to estimate the frequency offset and to compensate therefor. Differential detectors are described, for example, by Proakis in "Digital Communication," McGraw Hill, 1989, Chapter 4.2.6. However, differential detectors fail when the channel characteristics change rapidly.
To overcome the problem of both a frequency offset and a rapidly changing channel, practitioners have included a training word in the symbol sequence, and once the training word is detected and its position is ascertained, the frequency offset can be extracted. An algorithm for accomplishing this, which is quite complex, is presented, for example, by Bahai and Sarraf in "A Frequency Offset Estimation for Nonstationary Channels," Proc. of ICASSP 97, pp. 3897-3900, April, 1997.
A simpler solution would obviously be advantageous.